1. Field of the Invention
The present invention relates to a magnetic brush type developing apparatus and, more particularly, to a cavity preventing structure and residual developing agent amount detection in the magnetic brush type developing apparatus.
2. Description of the Prior Art
Typical examples of a developing apparatus using a dry developing agent are a magnetic brush type and a cascade type. In a magnetic brush type developing apparatus, a developing agent brush is formed on a developing sleeve by a magnetic field of a permanent magnet disposed inside the developing sleeve, and at least one of the developing sleeve and the permanent magnet is rotated while the magnetic brush is kept in contact with a photosensitive body, thereby developing an electrostatic latent image formed on the phtosensitive body. Since the magnetic brush type developing apparatus can be made compact, it is used more widely than a cascade type developing apparatus. In this magnetic brush type developing apparatus, in order to form a magnetic brush on the developing sleeve, a developing agent must always be in good contact with the sleeve surface. For this purpose, a signal representing a residual amount of the developing agent must be output or formation of a cavity in the developing agent in a developing vessel must be prevented. Thus, a conventional developing apparatus comprises a residual developing agent amount detecting means or a developing agent agitating mechanism.
Examples of a conventional agitating mechanism are a mechanism in which an agitating member (e.g., a screw) is rotatably disposed in a developing vessel and is externally rotated and a mechanism in which a cantilevered vibrating plate is disposed in a developing vessel and is vibrated utilizing rotation of a developing roller (consisting of a permanent magnet and a developing sleeve).
However, when the former mechanism is used as an agitating mechanism, one end of the agitating member must be projected outside the developing vessel and rotated by a belt or the like. Therefore, a bearing and a loop unit for transmitting power must be provided to the apparatus, resulting in a complicated structure and a large size of the apparatus. In addition, a problem of toner leakage from the bearing is posed. Moreover, if a drive force is externally applied, a rotational speed is maintained constant regardless of an amount of a developing agent. Therefore, if the developing agent amount is large, an excessive stress acts on the developing agent, thereby degrading the developing characteristics of the apparatus. On the other hand, when the latter mechanism is used as an agitating mechanism, an agitating effect is small, and cavities are easily produced.
In addition, a conventional residual developing agent amount detecting means utilizes an amount or the like of a developing agent or uses a piezoelectric vibrator and a Hall element, resulting in a complicated arrangement.